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Polymer Chemistry Polymer Chemistry View Article Online PERSPECTIVE View Journal | View Issue Dynamic covalent chemistry in polymer networks: a mechanistic perspective Cite this: Polym. Chem., 2019, 10, 6091 Johan M. Winne,*a Ludwik Leibler*b and Filip E. Du Prez *a The incorporation of dynamic covalent linkages within and between polymer chains brings new pro- perties to classical thermosetting polymer formulations, in particular in terms of thermal responses, pro- cessing options and intrinsic recycling abilities. Thus, in recent years, there has been a rapidly growing interest in the design and synthesis of monomers and cross-linkers that can be used as robust but at the same time reactive organic building blocks for dynamic polymer networks. In this perspective, a selection of such chemistries is highlighted, with a particular focus on the reaction mechanisms of molecular network rearrangements, and on how various mechanistic profiles can be related to the mechanical and Received 20th August 2019, physicochemical properties of polymer materials, in particular in relation with vitrimers, the recently Accepted 16th October 2019 defined third category of polymer materials. The recent advances in this area are not only expected to Creative Commons Attribution-NonCommercial 3.0 Unported Licence. DOI: 10.1039/c9py01260e help direct promising emerging polymer applications, but also point towards the need for a better funda- rsc.li/polymers mental understanding of chemical reactivity within a macromolecular context. Introduction up of a (highly) cross-linked macromolecular network, wherein the covalent links between or within polymer chains can be Classical polymer materials are expected to be chemically inert dynamically exchanged via a chemical reaction. Such dynamic and thus to have a fixed molecular make-up. Normally, chemi- covalent polymer materials can thus undergo a net process cal reactivity within polymers is considered as a nuisance that corresponding to a molecular network rearrangement (MNR). This article is licensed under a needs to be avoided or controlled with stabilizers. This situ- In a broader context, with focus on polymer applications, such ation has changed over the last few decades by a growing inter- polymer systems have also been defined as covalent adaptable est in the area of dynamic polymer systems, designed to networks (CANs),7,8,10 as such materials can ‘adapt’ their Open Access Article. Published on 16 10 2019. Downloaded 2021-10-02 12:36:15. achieve functional characteristics such as self-healing, shape- macromolecular architecture in response to an external stimu- – memory, repairing, stimuli-responsiveness and adaptability.1 8 lus or trigger, despite their cross-linked nature. The response Whereas these desirable polymer dynamics can be influenced or triggered behavior of such polymer materials will not only in many ways and in different contexts, such as in supramole- rely on the structural features of the polymer matrix but cular systems, one particular emerging strategy in this area has will also strongly depend on the rate and nature of the been to deliberately incorporate some degree of chemical reac- chemical bond exchanges within the polymer matrix. Thus, tivity into the polymer chains. Thus, many recent studies have the molecular mechanisms of these macromolecular rearrange- focused on the design of macromolecular matrices with ment reactions require close attention within the study – exchangeable, reversible or adaptable covalent bonds.1,2,7 9 In of such materials, as they can affect the mechanical material light of these developments, chemical reactivity considerations properties. can no longer be ignored in considering the physical pro- Building a fundamental understanding of the variables that perties of such polymer materials, and often even take on a govern material properties of dynamic polymer networks or dominant role. rearranging polymer networks should provide further gui- In this perspective paper, we would like to focus on one par- dance for the design of new materials and for the selection ticular subclass of dynamic polymer materials, i.e. those made and optimization of suitable building blocks. Herein, we provide the reader with a selection of exchange chemistries that have been explored in this regard, with a focus on their aPolymer Chemistry Research Group and Laboratory for Organic Synthesis, reactivity and mechanistic profiles, pointing towards opportu- Department of Organic and Macromolecular Chemistry, Ghent University, nities both in material as well as chemical science. For recent Krijgslaan 281 S4-bis, B-9000 Ghent, Belgium. E-mail: [email protected], overviews of such chemistries with a focus on their diversity [email protected] bUMR Gulliver 7083 CNRS, ESPCI Paris, PSL Research University, 10 rue Vauquelin, and applications in polymers, we refer to the reviews by Zhao 11 12 75005 Paris, France. E-mail: [email protected] and Xie, and by Konkolewicz. This journal is © The Royal Society of Chemistry 2019 Polym. Chem.,2019,10,6091–6108 | 6091 View Article Online Perspective Polymer Chemistry Before discussing various types of network rearrangement ations on the facility with which various shapes and dimen- reactions, we first provide an overview of how synthetic sions can be given to such materials. Once this final shape is polymer materials have been categorized in terms of properties chemically fixed, the material becomes unprocessable. that are related to their macromolecular architecture, and then Vitrimer polymers are the third category of organic polymer – how the categories of thermoplastic and thermosetting poly- materials, introduced by Leibler and coworkers in 2011.14 16 mers13 have been refined and even expanded with a third cat- When heated, vitrimers behave like a viscoelastic liquid, but – egory, vitrimers,14 16 when taking into account dynamic when exposed to solvent they will only show a limited soluble covalent macromolecular architectures. fraction, related to defects and unattached chains. Vitrimers derive these properties from a covalent molecular network that can change its topology through molecular rearrangements, Thermoplastics, thermosets, and while preserving the total number of bonds in the network, i.e. vitrimers the network connectivity. When a vitrimer is heated above its glass (or melting) temperature, molecular rearrangements can At low temperatures, polymer materials are either glassy or par- take place and the topology of the network fluctuates. Thus, tially crystallized and behave like hard solids. Found on their network segments can diffuse even though the network pre- response to temperature or solvent, three categories of serves its connectivity while spanning the volume of the whole polymer materials can be distinguished. sample. In terms of material properties, the permanent nature Thermoplastic polymers consist of linear or branched of the network enhances the resistance to polymer wear pro- polymer chains. When enough thermal energy is provided, cesses such as dissolution, creep or solvent stress cracking. thermoplastic polymers melt and the chains will be able to The dynamic nature of the network facilitates processing and diffuse. Thus, when heated above a certain temperature level, reprocessing and brings recycling possibilities despite the thermoplastic polymers behave like a viscoelastic liquid and presence of permanent covalent cross-links. Creative Commons Attribution-NonCommercial 3.0 Unported Licence. flow. When cooled, the material solidifies again. This fact In a broader context, vitrimers are a very specific type of allows for the typical ‘plastic’ continuous processing options dynamic polymer networks. It is important to stress at this point that have made (thermo)plastics into the most popular the fundamental difference between permanent dynamic net- materials for rapid production of consumer goods of various works (i.e. vitrimers) and reversible dynamic networks. Reversible shapes and dimensions. dynamic polymer networks contain non-permanent covalent When exposed to good solvents (and heat), thermoplastic links or cross-links that are able to break and reform through polymers will completely dissolve. reversible chemical bonding reactions. Such systems have been – Because of their macromolecular character, molten thermo- studied theoretically and experimentally for decades,17 20 and plastics behave like viscoelastic liquids, especially when chains have recently received a great deal of renewed attention from This article is licensed under a – are long and entangled. both academic and industrial research groups.1 12 Thermosetting polymers,13 on the other hand, when heated When heated, reversible polymer networks will undergo above their glass or melting temperature become softer, but topology fluctuations with fluctuations in overall connectivity, Open Access Article. Published on 16 10 2019. Downloaded 2021-10-02 12:36:15. they do not flow. Thermosetting polymers derive their endur- whereas in vitrimers, the postulated permanent nature of the ing and wide-ranging resistance to deformation from their network implies that the connectivity remains unchanged at (densely) cross-linked macromolecular structure, encompass- all temperatures and during the entire experimental time. This ing a macroscopic covalent network which spans the whole distinction is important because it will determine to some volume of the material. Segmental motions are hindered by extent the macroscopic properties that can be expected for connectivity
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